From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH v7 1/3] tcm_vhost: Introduce tcm_vhost_check_feature()
Date: Thu, 18 Apr 2013 10:44:00 +0300
Message-ID: <20130418074400.GH13787@redhat.com>
References: <20130417131726.GA26157@redhat.com>
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To: Asias He <asias@redhat.com>
Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, target-devel@vger.kernel.org, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>
List-Id: virtualization@lists.linuxfoundation.org

On Thu, Apr 18, 2013 at 04:25:21PM +0800, Asias He wrote:
> On Thu, Apr 18, 2013 at 10:06:06AM +0300, Michael S. Tsirkin wrote:
> > On Thu, Apr 18, 2013 at 09:05:52AM +0800, Asias He wrote:
> > > This helper is useful to check if a feature is supported.
> > > 
> > > Signed-off-by: Asias He <asias@redhat.com>
> > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > Why do we need the locking here by the way?
> > It seems features don't change while vhost
> > is running.
> 
> See:
> 
> static inline int vhost_has_feature(struct vhost_dev *dev, int bit)
> {
>         unsigned acked_features;
>         
>         /* TODO: check that we are running from vhost_worker or dev mutex is
>          * held? */
>         acked_features = rcu_dereference_index_check(dev->acked_features, 1);
>         return acked_features & (1 << bit);
> }       
> 
> We are accessing it outside the vhost_worker thread.

Hmm I see, we allow changing features on the fly, and the
reason is the LOG_ALL feature.

Okay ... do we ever call this from worker
thread? Because if we do, this will deadlock.
Maybe add a comment about this.

> > > ---
> > >  drivers/vhost/tcm_vhost.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > index 957a0b9..88ebb79 100644
> > > --- a/drivers/vhost/tcm_vhost.c
> > > +++ b/drivers/vhost/tcm_vhost.c
> > > @@ -99,6 +99,18 @@ static int iov_num_pages(struct iovec *iov)
> > >  	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> > >  }
> > >  
> > > +static bool tcm_vhost_check_feature(struct vhost_scsi *vs, int feature)
> > > +{
> > > +	bool ret = false;
> > > +
> > > +	mutex_lock(&vs->dev.mutex);
> > > +	if (vhost_has_feature(&vs->dev, feature))
> > > +		ret = true;
> > > +	mutex_unlock(&vs->dev.mutex);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > >  static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> > >  {
> > >  	return 1;
> > > -- 
> > > 1.8.1.4
> 
> -- 
> Asias